346 GENERAL ZOOLOGY 



food. It will leave the shark to seize a baited hook and is 

 frequently caught in that manner. All the benefit of the associ- 

 ation seems to be in favor of the suckfish. Sycotypus canalicula- 

 tus is a large, marine, gasteropod mollusc, common on the New 

 England coast. In the respiratory chamber of this animal one 

 frequently finds one or more flatworms, Planocera inquilina, 

 about x /i in. in length. They can be readily picked off the surface 

 of the respiratory chamber and so far as is known cause no injury 

 to the host. The benefit is again one-sided and in favor of the 

 flatworm, which gains food and shelter by the association and 

 gives nothing in return. Commensal associations occur fre- 

 quently and may represent a step toward a parasitic association, 



Fig. 196. — Remora, the suckfish. The dorsal fin is modified into a sucking disk 

 by which the fish attaches itself to a shark. {After Jordan and Kellogg.) 



in which the host definitely pays for the entertainment of the 

 guest. 



Symbiosis. — The living together of two different kinds of 

 organisms may develop to the point where the union becomes 

 very close without either suffering harm from the presence of the 

 other. This is known as symbiosis. Usually there is a mutual 

 benefit from the association. The hermit crab, Eupagurus, is 

 always found inhabiting an empty gasteropod shell, its abdomen 

 having become modified and slightly crowded to conform with 

 the spiral passage of the shell. In feeding or walking, only the 

 head and thorax protrude from the mouth of the shell. It can 

 retract into the shell and block the opening with an enlarged 

 claw (Fig. 197). When the shell becomes too small, the crab 

 seeks a larger one, sometimes killing and removing the original 

 inhabitant. In some species the surface of the shell is covered 

 with sea anemones or hydroid polyps which serve to conceal the 

 crab and also to protect it from fishes, because the polyps are 

 armed with stinging cells that make them unpalatable. When 

 the crab feeds, the polyp shares the food, capturing fragments as 

 they float upward or bending over the edge of the shell to reach 

 them. Evidently each animal benefits by the association. If 



